The present invention relates to optical fiber amplifiers, and more particularly to an efficient bi-directional optical fiber amplifier.
The realization of an optimized high efficiency bi-directional optical fiber amplifier is essential for the development of long range fiber optic data links ("FODL") for powered weapon system guidance. Applicable systems include air-to-ground or surface-to-surface missiles of extended range as well as long range land combat missiles. One exemplary such application is described in U.S. Pat. No. 5,005,930, for "Multi-Directional Payout Fiber Optic Canister," D. K. Schotter, and assigned to a common assignee with the present application. The entire contents of that patent are incorporated herein by this reference. Other potential applications for military systems include unmanned ground vehicle (UGV) and unmanned undersea vehicle (UUV). Potential commercial applications include long range radar and bi-directional satellite ground station relay links, CATV head-end link and fiber to the home (FTTH) fiber optics systems.
Erbium doped fiber (EDF) can be used to amplify optical signals in the 1.53-1.58 .mu.m band by converting pump lasers operating at a wavelength of 1.48 .mu.m or 0.98 .mu.m into signal power. For a long haul bi-directional fiber optics link, typically over 100 kilometers in length, a prior design of an optical signal amplifier operates by a single strand of EDF and suffers possible input signal level dependence, near-end reflection and in-line optical feedback induced noise, as well as cross-channel cross talk.
FIG. 1 illustrates a design for an EDF bi-directional optical fiber amplifier 20 employing a single strand of EDF, of the type described in commonly assigned pending application Ser. No. 07/655,615, filed Feb. 15, 1991, entitled "Amplifier for Optical Fiber Communication Link," by Hui-Pin Hsu, Ronald B. Chesler and Gregory L. Tangonan, now abandoned, the entire contents of which are incorporated herein by this reference. This design employs two signals and two diode laser pumps (22 and 24) coupled together by couplers 28 and 30, propagating in the same EDF gain medium (EDF 26). In the system of FIG. 1, an input signal at a first wavelength .lambda..sub.1 from a first transmitter enters a wavelength division multiplexing ("WDM") coupler 10, and is sent via a long length of optical fiber on a fiber bobbin 12 to the amplifier 20. The amplifier 20 amplifies the signal at the first wavelength, and sends it through another long length of optical fiber on a second fiber bobbin 16 to a second WDM coupler 18. The coupler 18 sends the signal at the first wavelength through a narrow band optical filter 19 centered at .lambda..sub.1, and on to a first receiver. A second transmitter sends a second input signal at a second wavelength .lambda..sub.2 into the second WDM coupler 18 and through the second long length of optical fiber on bobbin 16 to the bi-directional amplifier 20. The amplifier 20 amplifies the second input signal and sends it via the optical fiber on the first bobbin 12 to the first WDM coupler 10, which separates this signal from the first input signal and sends it via a narrow band optical filter 14 centered at .lambda..sub.2 to the second receiver. The filter 14 rejects light at the first wavelength .lambda..sub.1 ; the filter 19 rejects light at the second wavelength .lambda..sub.2.
The first transmitter, second receiver, optical filter 14 and WDM coupler 10 may be carried by the master vehicle, e.g., a manned airborne vehicle, and the second transmitter, first receiver, the WDM coupler 18, and the filter 19 can be carried on a slave vehicle such as a missile.
Drawbacks of the single channel EDF design of FIG. 1 include:
1) It is difficult to optimize the EDF length to accommodate wide input signal variations which dictate the EDF length for maximum signal gain. When the EDF 26 is too long, unwanted attenuation reduces the signal gain. When the EDF length is too short, both amplified signals will not be able to realize the full signal gain allowed. In contrast, the new dual-arm EDF design in accordance with this invention allows each EDF to be optimized individually for one signal channel for maximum saturable gain. This makes the optimum length of EDF less sensitive to the input signal level. PA1 2) Single strand EDF amplifier designs cannot provide the switch-off option for individual signal channels for the bi-directional link. PA1 3) Single strand EDF amplifiers will incur higher noise figure (NF) due to the presence of a co-directional pump. PA1 4) Single strand EDF amplifiers will also suffer cross-channel cross talk when one of the signal channels is operated at a low data rate of less than 100 Hz, due to the depletion of excited state population for the stimulated emission signal gain.
It is therefore an object of the present invention to provide a bi-directional EDF optical amplifier employing two separate EDF channels for counter-propagating signals to allow the optimization of each individual signal channel for efficient bi-directional optical signal repeater performance.